1. Field of the Invention
The present invention relates to rear wheel suspensions for bicycles. In particular to such suspensions that mount the rear wheel on a swing arm so that, in response to vibration and shocks, the wheel is able to move along a path relative to the bicycle frame against counteracting forces applied to the swing arm by a shock absorbing mechanism.
2. Description of Related Art
Rear wheel suspensions for bicycles of the initially-mentioned type have been known for over a century. While such suspensions have taken on numerous forms (see, e.g., British Patent No. 3982, German Patent No. DE 40 41 375, and U.S. Pat. Nos. 392,523; 423,471; 463,710; 465,599; 2,756,071; 3,982,770; and 5,335,929), constant factors have been the fact that wheel movement has been controlled to move in an arcuate path, in most cases being dictated by the presence of a fixed, single (real or virtual) pivot connection between the frame and the suspension to which the rear wheel is mounted, and the fact that bicycles with these suspensions never were able to find more than limited public acceptance.
The reasons why the prior rear wheel bicycle suspensions never attained wide public acceptance, despite their ability, to a greater or lesser extent, to effectively absorb shocks and vibrations, lie in the fact that they introduced other behavior characteristics that were more disturbing than the shock and vibration problems they solved. That is, unless the pedal crank pivot was mounted at or near the pivot connection of the swing arm to the frame, the vertical component of the swing arm movement would adversely impact on the pedaling "feel" and the rider's ability to effectively apply a constant force to the pedals during riding. Furthermore, since the pedals, and therefore the pedal crank pivot, must be located near the middle of the bicycle (underneath and slightly forward of the seat), a bicycle with such a suspension produces a center hinge effect which leads to several problems affecting riding comfort and performance. In particular, a tendency exists for the front half (main frame) of the bicycle to rock about the pivot connection between it and the swing arm. This rocking movement changes the head angle and is perceived as a bobbing effect similar to riding a children's "rocking horse". Moreover, the suspension can act to absorb a portion of the pedal forces, affecting performance, and this absorption is translated into movement of the suspension, again, affecting riding "feel". Similarly, application of braking forces to the rear wheel would, in reverse, be transmitted into the suspension causing the rider to experience a "sinking" effect. Existing systems have faced have faced one or more other problems as well including traction and braking inconsistencies, and handling inconsistencies under competition conditions, to name just a few.
With the advent of professional bicycle racing, not merely road or track racing, but mountain or dirt bike racing, cross country and downhill (where bicycles travel downhill over rough terrain at speeds of around 40 mph (67 kph)), the demand for high performance bicycle rear suspensions has increased, while the problems of prior bicycle suspensions have been amplified under such racing conditions. That is, the forces to which the suspension is subjected require increased wheel travel to absorb the induced shocks as well as the need to use softer springs for bump compliance (i.e., so that the wheel will follow the bump instead of bouncing off it) for traction purposes. However, changes of this type make previously "invisible" force problems not only apparent, but unacceptable. Put another way, as wheel travel increases, the importance of maintaining consistent (and constant) force functions within the bicycle-rider system increases and this has been obtainable, to date, in most rear suspensions only by the use of very stiff springs to correct for geometry induced problems (many suspensions also limit downward wheel travel from static ride height to zero), thereby sacrificing bump compliance.
In contrast to bicycles, motor vehicle, and particularly motorcycle, rear (drive) wheel swing arm suspensions have been developed which do not use a single pivot motion mechanism. For example, in U.S. Pat. No. 4,735,277, motorcycle drive wheel suspensions are disclosed which use at least two swing arms, one of which is connected to the cycle frame and a second of which is connected to the wheel, in a way which permits the rear wheel to freely move in any direction relative to the frame within the plane of rotation of the wheel and allowing the wheel to move along a plurality of paths. However, considering that this patent contains no disclosure as to what purpose is served by permitting the rear wheel to freely move in any direction relative to the frame within the plane of rotation of the wheel and allowing the wheel to move along a plurality of paths, and given the performance, applied load and ride differences between motorcycles and bicycles, not to mention the presence of a frame-mounted motor instead of a suspension-mounted pedal crank, no practical means or reason to apply such a suspension to a bicycle can be derived from this patent.
Likewise, U.S. Pat. No. 4,671,525, discloses a suspension for the rear wheels of motor vehicles in which the wheel-carrying swing arm forms part of a quadrilateral linkage assembly in which the shock absorbing members can be located between any members and the geometry of the articulated system can be designed to produce movement of the rear wheel along any desired path, the described embodiment attaining an almost linear or slightly curved path having a substantially vertical, upward and rearward inclination. However, while the advantages of this suspension, in addition to being able to be adapted to produce any desired path of movement, are indicated as including its ability to counteract "sinking during an acceleration" and "raising when braked," due to its ability to perform a long elastic excursion, no particular significance is attached to any particular linkage configuration or resultant path of movement relative to this advantage or any other. Thus, since this patent also relates to motor vehicles having a frame-mounted motor instead of a suspension-mounted pedal crank, no practical means or reason can be obtained from this patent to apply such a suspension to a bicycle, again, recognizing the differences in performance, ride and applied forces occurring in the motorcycle context in comparison to bicycles.
Thus, a need still exists for a rear wheel suspension for bicycles which will overcome the above-mentioned problems associated with bicycle swing arm suspensions as they have been constructed to date, and no means to fill that need frown existing motorcycle rear wheel suspensions being apparent. In particular, a need exists for a bicycle rear wheel suspension which will meet the needs of competitive mountain bike racing.